CEPHALOPODA. 327 



gives rise to the two rounded convexities observable on the ventral 

 aspect of the body behind the funnel. It is a transversely oblong 

 mass, composed of numerous close-set pectinated membranous la- 

 minse, which are about a quarter of an inch in depth, and are dis-^ 

 posed in three groups ; those of the larger group extend transversely 

 across the mesial line of the body, and are unprotected by a mem- 

 brane ; but the two smaller divisions are symmetrically disposed, and 

 have the unattached edges of the laminae covered by a thin membrane, 

 which is reflected over them from the anterior margin of the glandular 

 bodj\ These divisions form the sides and anterior part of the gland ; 

 and as the secreted matter must pass backwards to escape from be- 

 neath the margin of the protecting membrane, this membrane may 

 serve both to conduct the secretion nearer the orifice of the oviduct, 

 and also to prevent its being drawn within the respiratory currents of 

 water, and so washed away as soon as formed. 



In contrasting the organisation of the Nautilus with that of 

 the inferior Mollusca treated of in the two preceding lectures, we 

 find the main advance to have been made in the organs of animal 

 life. 



A true internal skeleton is established in the Nautilus, and thus the 

 lowest Cephalopod offers an approximation to the Vertebrate type, 

 which not even the highest of the Articulate series had attained. 

 Perfect symmetry now reigns throughout the animal and vital organs. 

 The muscular system forms a larger proportion of the body, with various 

 arrangements and complications unknown in the lower Encephalous 

 moUusks. The respiratory tube, though still completed by the over- 

 lapping, not by the coalescence, of its side-walls, has received an 

 enormous development as contrasted with the siphonated Trachelipods; 

 and, by its powerful muscles and their firm cartilaginous basis of 

 attachment, it is evidently endowed with a new function, in relation 

 to propelling the Cephalopod with its testaceous dwelling through the 

 sea. 



The nervous centres concentrated in the head have received a marked 

 increase of bulk, which, nevertheless, is still manifested more strongly 

 in the inferior masses, and especially the anterior sub-cesophageal ring 

 than in the superior or cerebral part. Here, however, we find for the 

 first time in the Molluscous series, especial ganglions subordinated to 

 the greatly enlarged organs of vision. 



The organs of reptation, which had progressively advanced (as La- 

 marck's denomination of the higher Gasteropods indicates) towards 

 the head, are exclusively attached to that part in the Nautilus, and 

 project from before the eyes and mouth. The mouth, besides its jaws 

 and spiny tongue, is now served by organs of prehension; and it is 



Y 4< 



